High speed tracked vehicles are often designed with large numbers of roadwheels to maximize the wheel base and reduce the amplitude of undesired pitching. However, the relatively long wheel base conflicts with easy turning action achieved by a shorter wheel base. The present invention is directed to an auxiliary power mechanism which automatically reduces the length of track subjected to high ground pressures during vehicle turning maneuvers; the reduced effective track length enables the vehicle to undergo a desired turn action with minimum power expenditure and lessened wear on the track components. This increased agility of the vehicle will be achieved without sacrifice in rideability or vehicle speed under normal straight-ahead operations.